CD52 is a GPI-anchored glycoprotein restricted to the male reproductive tract and lymphocytes. Although the structure of CD52 has been extensively characterized and the function of lymphocyte CD52 has been investigated, the role of CD52 in sperm maturation and function is unknown. On the lymphocyte surface, CD52 is associated with the T cell receptor and is present in protein complexes containing tyrosine kinase activity; cross linking of CD52-containing complexes were shown to trigger protein tyrosine phosphorylation and induce T cell activation. Thus, CD52 is indicated in signal transduction events associated with TCR-dependent activation of lymphocytes. We hypothesize that sperm-surface CD52 exhibits similar signal transduction-related functions and complex-forming properties as those identified in the lymphocyte. In spermatozoa, insights into the functional role of CD52 have been obtained using S19, a monoclonal antibody (mAb) specific for sperm CD52. S19 Fab fragments were shown to inhibit human sperm-zona pellucida tight binding implicating CD52 itself or a CD52-containing complex in sperm-zona interactions. Preliminary results indicate that incubation of human spermatozoa with the bivalent S19 mAb promotes the acrosome reaction (AR) while monovalent S19 Fab does not. These results suggest that CD52 aggregation has a role in induction of the sperm AR, a signal transduction-mediated event that occurs upon sperm-zona pellucida binding. Collectively, results from sperm and lymphocyte studies implicate a functional role for CD52 in sperm maturation and/or function. To test our hypotheses, the specific aims of the proposed research project are to investigate the function of CD52 in the human sperm AR and to identify proteins associated with sperm CD52. The capacitation- and calcium-dependence of the CD52-induced AR will be determined. Sensitivity to inhibition by pertussis toxin will be examined to determine whether the observed AR occurs via a G protein-mediated pathway. Changes in protein tyrosine phosphorylation upon cross linking of sperm- surface CD52 will be determined. Furthermore, CD52-associated sperm proteins will be isolated and microsequenced. The identified CD52-associated proteins will represent candidate molecules for signal transduction events involved in sperm function and for sperm-zona interactions. These studies will provide insights into the function of sperm CD52 and into the signal transduction mechanisms involved in the AR.